The flurry of diplomatic activity comes in what US Treasury Secretary Timothy Geithner described at Wednesday’s meeting in Washington as “the shadow of a massacre” after last month’s killings of more than 100 civilians in the city of Houla.

But the calls for a tightening squeeze on Assad also continue to hit the by-now familiar wall of resistance from Russia and China, which oppose additional measures against the Syrian regime. The two powers issued a statement Wednesday rejecting any outside efforts to promote regime change in Syria.

At the Washington meeting of the Friends of the Syrian People International Working Group on Sanctions, Secretary Geithner called on countries that have not yet imposed sanctions on Syria to do so, and he promoted stepped-up coordination of national sanctions as a means of tightening the noose on Assad.

Geithner also laid out on the table the eventual adoption by the United Nations Security Council of measures authorizing the use of international force in Syria – as recommended, Geithner noted, by the Arab League last weekend. But that step appears quite unlikely, given that both Russia and China hold veto power over Security Council action.

The Washington meeting seems likely to have further antagonized Russia in particular, even as the Istanbul meeting is expected to explore ways to enlist Russia’s assistance in encouraging a political transition in Syria.

Arriving in Istanbul Wednesday, Secretary of State Hillary Rodham Clinton said the countries meeting Thursday would consider ways to pressure Assad to “allow the transition to a democratic Syria to begin.”

In a statement issued after its meeting, the sanctions group called on countries to “harmonize national and regional sanctions regimes” by imposing an asset freeze on senior Syrian regime officials, and an asset freeze on the Central Bank of Syria and the Commercial Bank of Syria “to ensure their isolation from the international financial system.” The group also called for an embargo on Syrian petroleum products and a ban on insurance for shipments of Syrian petroleum products.

A senior Treasury official speaking after the meeting declined to speculate on the immediate impact it would have in terms of ratcheting up pressure on Assad, but he said that the number of countries attending suggested a growing interest in international action on Syria.

The meeting indicated an “increasing recognition of the role well-designed sanctions can play in a conflict like this,” he said.

The group also called on all members of the international community to adopt an embargo on the supply of arms to the Syrian regime – a measure that seemed pointed at Russia in particular.

Last week, the group Human Rights First reported the arrival in Syria of a Russian ship laden with arms, including heavy weapons. At the United Nations, the US ambassador to the UN, Susan Rice, called Russia’s continued arming of the Assad regime “reprehensible.”